While operating a commercial vehicle, a driver notices that the air pressure in the secondary (front axle) air tank is significantly lower than the primary (rear axle) air tank, despite the compressor running normally. What is the most probable cause?
- AA leak in the secondary air tank supply line or the tank itself.
- BThe air compressor is only supplying air to the rear axle brakes.
- CThe safety relief valve on the primary air tank is stuck open.
- DA clogged air filter in the primary air tank.
Show answer & explanationAnswer & explanation
Correct answer: A. A leak in the secondary air tank supply line or the tank itself.
In a dual air brake system, if one tank's pressure is significantly lower than the other while the compressor is functioning, it strongly suggests a leak in the specific system that is losing pressure. The secondary air tank supplies the front axle brakes, and a leak in its supply line or the tank itself would cause this differential pressure loss.
Why the other options are wrong
- B. The air compressor supplies both tanks, not just one. If it were only supplying one, the other would not build pressure at all, not just be lower.
- C. A stuck-open safety relief valve on the primary tank would cause the primary tank to lose pressure, not the secondary.
- D. A clogged air filter in the primary tank would not directly cause a pressure drop in the secondary tank.
Dual Air System Pressure Imbalance
In a dual air brake system, if one air tank consistently shows lower pressure than the other while the compressor is working, it indicates a leak or restriction specifically within the low-pressure system (tank or supply lines).
- Dual systems have separate tanks for front and rear axles.
- Compressor supplies both tanks.
- Pressure difference points to an issue in the lower-pressure circuit.
Memory trick: If one 'lung' of the dual air system is deflating faster, it must have a hole or a pinched tube.